In recent years, the greenhouse effect due to CO2 is being pointed out as one of causes of the global warming phenomenon, and measures against it are urgently needed in view of the global environmental protection. Since a major CO2 generating source is found in areas of human activity where the fossil fuel is burnt, there are growing demands for limitation of CO2 emission. Accordingly, a method of recovering CO2 by removing it from the combustion exhaust gas by contacting the combustion exhaust gas to an absorption liquid has been studied vigorously for a thermal power plant using a large amount of fossil fuel.
As an example of the CO2 recovery system, there is a known CO2 recovery system (Research Institute of Innovative Technology for the Earth, 2006 Achievement Report). In a CO2 recovery system, the combustion exhaust gas containing CO2 to be recovered is guided to an absorption tower through a combustion exhaust gas supply line. Counterflow contact occurs between an absorption liquid supplied through an absorption liquid supply line and the combustion exhaust gas in the absorption tower, and CO2 contained in the combustion exhaust gas is absorbed for removal by the absorption liquid. The combustion exhaust gas from which CO2 has been removed goes upward, and it is exhausted through a combustion exhaust gas discharge line.
The absorption liquid having absorbed CO2 is sent to a heat exchanger via a discharge pump through an absorption liquid discharge line, heated therein and further guided to a regeneration tower 32. The absorption liquid guided to the regeneration tower is heated for regeneration by a regenerative heater such that CO2 is released and separated. The absorption liquid regenerated by the regeneration tower is guided again to the absorption tower by a supply pump. In this case, the temperature of the absorption liquid is adjusted by the heat exchanger or an absorption liquid cooler which is disposed if necessary.
And, CO2 separated from the absorption liquid within the regeneration tower is cooled together with steam, which is generated when the absorption liquid is heated by the regenerative heater, by a reflux condenser. Subsequently, CO2 and condensed water are separated from each other by a separator, and CO2 from which steam has been removed is guided to a CO2 recovery process through a recovery line. The separated condensed water is returned to the regeneration tower by a reflux pump.
As the absorption liquid in the CO2 recovery system described above, many aqueous solutions containing an amine are known. JP-A 61-71819 (KOKAI) describes an acid gas scrapping composition containing nonaqueous solvents such as a sterically hindered amine and sulfolane, and 2-amino-2-methyl-1-propanol and the like as a sterically hindered primary monoamino alcohol.
JP-A 5-301023 (KOKAI) describes a particular aqueous hindered amine solution having as a typical example a hindered amine selected from a group consisting of 2-amino-2-methyl-1-propanol, 2-(methylamino)-ethanol, 2-(ethylamino)-ethanol, 2-(diethylamino)-ethanol, and 2-(hydroxyethyl)-piperidine.
JP-A 8-252430 (KOKAI) describes an amine mixed solution which has secondary and tertiary amines each having a concentration in a range of 10-45 wt %. A CO2 absorption behavior of an aqueous solution of 2-amino-2-2 methyl-1-propanol which is a hindered amine is disclosed in Chemical Engineering Science, Vol. 41, No. 4, pp. 997-1003.
In a case where the above absorption liquid is used, it is preferable that the absorption liquid is cooled to a given level and introduced into the absorption tower because the concentration of CO2 in the absorption liquid becomes high when its temperature is lower. And, since the release of CO2 is an endothermic reaction, the regeneration tower needs to heat the absorption liquid.
But, the release of CO2 by the regeneration tower requires an enormous amount of thermal energy, which contributes to the high cost when CO2 is recovered and becomes a major hurdle in view of practical realization of the CO2 recovery system. In other words, when the absorption liquid is heated in order to release CO2, 50 mass % or more of water generally contained in the absorption liquid is evaporated at the same time, and its evaporative latent heat is high. Therefore, about a half of the thermal energy supplied from the regenerative heater and the like is consumed for evaporating the water.
Meanwhile, the amine in the absorption liquid is hydrated with water molecules by a hydrophilic group, and OH groups present at this time play an important role in bonding with CO2, and more specifically with CO32−. Therefore, to provide the functions as the absorption liquid, it is necessary to have some water in the absorption liquid, and it is hard to eliminate water completely.